|△a contraction of “am not,” “is not,” “are not,” “have not,” or “has not” (nonstandard)|
Ain’t is one of the most informal verb contractions in English, and its use in formal contexts may be criticized because it is associated with careless speech. It is, however, accepted in folk and popular song lyrics, show titles, direct quotations, and fictional dialogue. Otherwise ain’t is best avoided, except as a deliberate rhetorical device and in allusive expressions such as You ain’t seen nothing yet.
<taken from Microsoft Encarta Dictionary>
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